I did a quick search and found some tutorials that I think are useful. For example this:
http://meisan.deviantart.com/art/contrast-digital-ink-tutorial-35973729
This sums up what I mean by "good picture even without screentone". I know, it is difficult in the beginning to be bold with black colour but have courage and try things out.
smile Don´t fear using black. It not only looks good on hair but on clothes also. And shadows. And structure!
wink
Here is a tutorial for Photoshop:
http://pandabaka.deviantart.com/art/How-to-basic-screentone-on-PTS-171673878
And here one for Manga Studio:
http://yuurei-onnanoko.deviantart.com/art Www/Screentone-Tutorial-Prt-1-186970131
Finally, here is a tutorial for digital screentone, contrasts and other stuff:
http://minyi.deviantart.com/art/Digital-Manga-Tutorial-134439901
If I were you I would really work on the inking first before trying to learn screentone. At the moment you use outlines of the same thickness with a lot of white and just a little black. Turn the game around!
biggrin Imagine that you are sitting in front of a black canvas and you have to paint in white. Also I would advise you to try to use lines of different shape and thickness. Or even dots! And lots of structure and patterns! Like this:
http://www.artgraphica.net/free-art-lessons/pen-and-ink-tutorial.html
http://www.artinstructionblog.com/how-to-draw-leaves-on-a-tree-pen-drawing-tutorial
http://www.kunstkurs-online.de/Seiten/zeichnen-technik/tusche-zeichnen.php